The present invention relates to the positioning of a sensing device over a data record member and, more particularly, to a technique for enabling such device to follow accurately a desired track on the record member containing data to be retrieved.
In the field of data processing, use is frequently made of storage systems for storing large quantities of data. One form of these storage systems includes magnetizable rotating disks which have information placed serially on a plurality of concentric tracks located on each disk. This format of data storage is used in random access systems in which a transducer or head is positioned with respect to a desired track to reproduce or record the information.
To obtain random access to the information stored on the plurality of concentric tracks, means must be provided for positioning accurately the recording and reproducing head over the track containing the desired information. Prior to the introduction of high density disks having, for example, about 200 tracks per inch, it was sufficient to position the head merely in the vicinity of the desired track, i.e. largely ignoring thermal effects and disk runout, to record or reproduce the data. This "coarse" positioning was accomplished with a transducer which was not a part of, and therefore external to, the recording and reproducing head, but which was located on a carriage for moving the head radially across the tracks. However, with the advent of the high density disks, it became necessary to position the recording and reproducing head more accurately, preferably exactly centering the head with respect to the desired track.
One system for centering such head over the track includes a disk pack having a plurality of axially aligned disks mounted on and rotated by a common spindle. Each of the disk surfaces, except one, has only recording data, while the one surface has dedicated to it only head positioning or servo data. Each disk surface has a head movable over it, all the heads being bolted to a common movable carriage. Therefore, one head, which is the servo transducer, is associated with the disk surface storing the servo data, while the other heads, which are recording or reproducing transducers, are associated with the surfaces storing the recording data. The servo transducer senses the servo data so that error signals can be generated when such transducer is not centered in relation to a desired track, these error signals causing a servo motor to move the carriage until the transducer is so centered. As a result, the recording and reproducing heads, which are aligned with the servo transducer, presumably also will be positioned in relation to the center of their corresponding tracks to, for example, reproduce recording data from a selected disk surface.
One disadvantage with such system having a dedicated surface of servo data is that all the heads must be accurately aligned with respect to each other so that, for example, when the servo head is centered on a desired track number 20, the recording and reproducing heads also are centered on track number 20 of their respective disk surfaces. However, this alignment or calibration is easily subjected to human error, thereby resulting in misalignment of the heads. Also, it is not unlikely that the runout or wobble of the disks in the pack may be different for each disk, so that even if all the heads are accurately aligned, when the servo head is centered on track 20, one of the recording and reproducing heads may be deviated from track center. Furthermore, this servoing system normally is used with a disk pack containing at least ten disks or twenty disk surfaces where the servo overhead is only one dedicated surface out of twenty. If one desires to use a disk pack containing only one disk, then there is a high servo overhead since fifty per cent of the disk surfaces is used for servo data.
A system which overcomes the above disadvantages comprises a recording and reproducing head which also functions as a positioning or servo transducer. In this system, each disk surface in the disk pack includes both servo data and recorded data. The servo data is stored in a plurality of radially extending sectors spaced about the disk surface and on the same tracks as the recorded data. As the head associated with each disk surface follows a desired track, it senses the recorded data, and when it traverses a sector, it detects the servo data to align itself with the center of the desired track.
The above system using a head for both servoing and recording or reproducing has its own disadvantages. First, since in this system the recording and reproducing head is also the servo head, there is a problem of channel dynamics. That is, because the channel including the head has to operate over a wide bandwidth to process both the low and high frequencies of the servo data and recording data, respectively, high frequency channel noise can cause aliasing problems. Furthermore, each such head for each disk surface may have its own offset; that is, each head acting as a servo transducer may provide slightly offset information as to the position of the head, thereby requiring separate calibration of each head to account for the offset. This offset might be due, for example, to a chip in the head gap, or the gap cross-section along the entire length of the gap not being uniform.
The servo data, which can be used for either the dedicated surface system or the interspersed servo and recording data system described above, can comprise, for example, one of two codes known in the art, respectively, as the dibit and tribit codes. These codes include magnetized regions on both sides of the centerline of each track. As the head follows the track, peak amplitudes of the rate of change of flux on both sides of the centerline of the track are detected. If the head is not centered in relation to the centerline of the track, these peak amplitudes will not be equal, and an error signal will be generated to energize a servo motor to move the head and center it on the track, at which time these peak amplitudes will be equal so that no error signal is generated. The primary difference between the tribit code and dibit code is that the former contains its own timing or synchronizing information resulting in one method of peak amplitude detection, while the latter does not and therefore requires a different method of detection.
The dibit and tribit codes, and their manner of detection, have their own disadvantages. First, even if the head is centered on the desired track, it is possible to detect incorrectly that the head is off center. This is because the code itself is not designed to take into account the above-mentioned problems of head construction. For example, if the cross-section of the gap of a transducer is not uniform, such that the gap on one side of the head is slightly wider than the gap on the other side of the head, then the codes are such that different peak amplitudes of the respective change in fluxes will be detected. Consequently, the servo motor will move the head off center, though the head was centered. The same result will occur if the flying height of the head, i.e., the height of the head above the disk, is different on either side of the centerline of the track, as is not unusual.
Furthermore, any system using the dibit or tribit codes requires two separate channels to detect, respectively, the peak amplitudes of rate of change of flux on the left side and right side of the centerline desired track. This means that the electronics in each channel must be accurately designed and calibrated in relation to each other so that if the head is on center, one channel will not incorrectly indicate a higher peak amplitude with respect to the other channel. Concomitantly, the use of these two separate channels for detecting the peak amplitudes means a higher cost for the entire system. Furthermore, since the system using a dibit or tribit code is a peak amplitude detection system, it is possible that channel noise having a peak amplitude higher than the peak amplitude of the rate of change of flux may be detected during the time of detecting, for example, the peak amplitude for the left side of the track, thereby falsely indicating that the head is off center.